Squirrel-cage induction motors are widely used in the agricultural, commercial, municipal, and residential sectors due to their high energy efficiency, reliability, and good controllability. As induction motors become more involved in critical tasks, such as heating, ventilating, and air conditioning (HVAC) systems used in places like hospital intensive-care units (ICU) and energy-efficient buildings, accurate and reliable condition monitoring of their status is assuming a greater importance. An important part of this condition monitoring task involves an accurate estimation of the induction motor inductance parameters, such as the stator inductance and the total leakage factor.
To obtain inductance parameters for an induction motor without interrupting normal motor operations, online and noninvasive inductance parameter estimation methods are typically in practice. Such methods estimate an in-service induction motor's stator inductance and total leakage factor based on the voltage and current measurements acquired at motor terminals or at motor control centers. There are various known methods for inductance parameter estimation for inverter-fed motors (i.e., motors that are connected to ac drives). Some such methods involve injecting certain signals into the motor, which require separate electronic circuits be inserted between the power supply and the motor. Although reasonably accurate estimates of induction motor inductance parameters are obtainable, it is rather impractical to implement these methods for line-connected motors, as these motors are connected directly to their power supplies, and normally there is no room for separate electronic circuits.
There are currently two primary approaches for obtaining inductance parameters in an online and noninvasive manner for line-connected induction motors. The first approach is based on the steady-state induction motor equivalent circuit model. By collecting voltage and current measurements from a line-connected induction motor operated at multiple distinct load levels, these methods estimate the induction motor electrical parameters, including inductance parameters, without actually stopping the motor. To simplify the estimation process, an assumption is made on the ratio of the stator leakage inductance to the magnetizing inductance. This approach may not work well for line-connected motors under dynamic motor operations, such as motors connected to time-varying loads like reciprocating compressors or pumps.
The second approach is based on the dynamic induction motor equivalent circuit model. This approach estimates the induction motor electrical parameters, including inductance parameters, by computing a least-squares solution. This technique requires an accurate knowledge of the instantaneous rotor speed. The rotor speed is obtained from an external speed sensor attached to the shaft of the motor. Such speed sensors are costly and fragile, and are very difficult to install in many motor applications.